package com.lj.infoisland.util;

public class SnowflakeIdGenerator {
    // 开始时间戳 (2020-01-01)
    private final long twepoch = 1577836800000L;
    
    // 机器ID所占的位数
    private final long workerIdBits = 5L;
    
    // 数据中心ID所占的位数
    private final long datacenterIdBits = 5L;
    
    // 支持的最大机器ID，结果是31
    private final long maxWorkerId = ~(-1L << workerIdBits);
    
    // 支持的最大数据中心ID，结果是31
    private final long maxDatacenterId = ~(-1L << datacenterIdBits);
    
    // 序列在ID中占的位数
    private final long sequenceBits = 12L;
    
    // 机器ID向左移12位
    private final long workerIdShift = sequenceBits;
    
    // 数据中心ID向左移17位 (12+5)
    private final long datacenterIdShift = sequenceBits + workerIdBits;
    
    // 时间戳向左移22位 (5+5+12)
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    
    // 生成序列的掩码，4095
    private final long sequenceMask = ~(-1L << sequenceBits);
    
    // 工作机器ID(0~31)
    private long workerId;
    
    // 数据中心ID(0~31)
    private long datacenterId;
    
    // 毫秒内序列(0~4095)
    private long sequence = 0L;
    
    // 上次生成ID的时间戳
    private long lastTimestamp = -1L;

    // 构造函数
    public SnowflakeIdGenerator(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        if (datacenterId > maxDatacenterId || datacenterId < 0) {
            throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
        }
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }

    // 生成唯一ID的方法
    public synchronized String nextId() {
        long timestamp = timeGen();

        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

        // 如果是同一时间生成的，则进行毫秒内序列
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            // 毫秒内序列溢出
            if (sequence == 0) {
                // 阻塞到下一个毫秒,获得新的时间戳
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            // 时间戳改变，毫秒内序列重置
            sequence = 0L;
        }

        // 上次生成ID的时间戳
        lastTimestamp = timestamp;

        // 移位并通过或运算拼到一起组成64位的ID
        long id = ((timestamp - twepoch) << timestampLeftShift)
                | (datacenterId << datacenterIdShift)
                | (workerId << workerIdShift)
                | sequence;

        // 转换为16进制字符串并填充到32位
        return String.valueOf(id);
    }

    // 阻塞到下一个毫秒，直到获得新的时间戳
    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    // 返回以毫秒为单位的当前时间
    protected long timeGen() {
        return System.currentTimeMillis();
    }

    public static void main(String[] args) {
        SnowflakeIdGenerator idGenerator = new SnowflakeIdGenerator(1, 1);
        for (int i = 0; i < 10; i++) {
            System.out.println(idGenerator.nextId());
        }
    }
}